The present invention relates to solar cells, and in particular to high efficiency, multijunction solar cells formed primarily of III-V semiconductor alloys.
The highest known solar cell efficiencies have been produced by multijunction solar cells comprised primarily of III-V semiconductor alloys. Such alloys are combinations of elements drawn from columns IIIA and VA of the standard Periodic Table, identified hereinafter by their standard chemical symbols, names and abbreviations, and wherein the total number of elements from column IIIA is substantially equal to the total number of elements from column VA. The high efficiencies of these solar cells make them attractive for terrestrial concentrating photovoltaic systems and systems designed to operate in space.
Historically, the highest efficiency solar cells have consisted of a monolithic stack of three subcells, which are equivalently referred to as junctions, grown on germanium (Ge) or gallium arsenide (GaAs) substrates. The subcells contain the regions of the solar cell where light energy in a range of wavelengths is absorbed and converted into electrical energy that may be collected externally. The subcells may be interconnected with one another by tunnel junctions. Other layers, such as buffer layers, may also exist between the subcells. In the highest efficiency solar cells demonstrated to date, the top subcell has one or more absorbing layers made of (Al)GaInP, the intermediate subcell has one or more absorbing layers made of (In)GaAs, and the bottom subcell includes a Ge substrate or has absorbing layers made of a III-V material. The foregoing nomenclature for a III-V alloy, wherein a constituent element is shown parenthetically, such as Al in (Al)InGaP, denotes a condition of variability in which that particular element can be zero.
Each subcell comprises several associated layers, typically including a window, emitter, base and back surface field (BSF). These terms are well known to those skilled in the art and do not need further definition here. Each of the foregoing layers may itself include one or more sublayers. The window and emitter will be of one doping polarity (e.g., n-type) and the base and back surface field will be of the opposite polarity (e.g., p-type), with a p-n or n-p junction formed between the base and the emitter. If the base contains an intrinsic region in addition to an intentionally doped region, then it may be considered a p-i-n or n-i-p junction, as is well known to those skilled in the art. By convention, the specific alloy and the band gap of a given subcell are considered to be the name and the band gap, respectively, of the material forming the base. This material may or may not also be used for the window, emitter and back surface field of the subcell. For example, a subcell comprising an AlInP window, an InGaP emitter, a GaAs base and an AlGaAs back surface field would be denoted a GaAs subcell and its band gap would be the GaAs band gap of 1.4 eV. A subcell comprising an AlInP window, an InGaP emitter, an InGaP base and an InGaP back surface field would be denoted an InGaP subcell, and its band gap would be that of the InGaP base. The subcell may include layers in addition to those listed above. Those skilled in the art will also recognize that subcells may also be constructed without one or more of the foregoing layers. For example, subcells may be constructed without a window or without a back surface field.
When speaking about the stacking order of the subcells from top to bottom, the top subcell is defined to be the subcell closest to the light source during operation of the solar cell, and the bottom subcell is furthest from the light source. Relative terms like “above,” “below,” “upper,” and “lower” also refer to position in the stack with respect to the light source. The order in which the subcells were grown is not relevant to this definition. The top subcell is also denoted “J1,” with “J2” being the second subcell from the top, “J3” being third from the top, and the highest number going to the bottom subcell.
Three junction solar cells have reached the highest efficiencies of any solar cells to date. See M. A. Green et al., Progress in Photovoltaics: Research and Applications 19 (2011) 565-572. However, these three junction solar cells are approaching their practical efficiency limits. To reach significantly higher efficiencies, additional junctions or subcells are needed. With additional subcells, photons can be absorbed more efficiently by materials with band gaps closer to the photon energies, which are able to convert more light energy into electrical energy rather than heat. In addition, the total solar cell current with additional subcells may be lower for a given amount of incident light, which may reduce series resistance losses. Another mechanism for increasing efficiency is to absorb a larger fraction of the solar spectrum with additional subcell(s). For many years, there has been widespread recognition of the need for higher numbers of junctions, but to date, the attempt to build cells of four, five and six junctions has failed to produce solar cells with efficiencies that exceeded the efficiencies of the best three-junction solar cells. The reasons for failure have been unclear, although material and design flaws have been suspected, including poor material quality, which can be a result of dislocations produced by the use of lattice-mismatched layers. There are additional challenges related to the increased number of tunnel junctions required to interconnect the additional subcells, including the loss of light by tunnel junction absorption.
There has long been interest in high efficiency, lattice-matched multijunction solar cells with four or more subcells, but suitable materials for creating high efficiencies while maintaining lattice matching among the subcells and to a substrate have previously been elusive. For example, U.S. Pat. No. 7,807,921 discusses the possibility of a four junction, lattice-matched solar cell with GaInNAs as the material for a 1.0 eV subcell. However, the applicant concluded that this design is impractical because GaInNAs that is lattice matched to the other subcells exhibited poor quality when produced by then known techniques. To overcome the problems with finding feasible, lattice-matched structures, the patent teaches the use of metamorphic materials including a graded metamorphic layer of GaInNAs that is not lattice matched. In another attempt to make a 1 eV subcell that may be lattice-matched to the traditional InGaP/(In)GaAs/Ge solar cell, a material consisting of gallium, indium, nitrogen, arsenic and various concentrations of antimony was studied, but these investigators concluded that antimony, even in small concentrations should be avoided as it was considered detrimental to device performance. See Ptak et al., Journal of Vacuum Science Technology B 25(3) May/June 2007 pp. 955-959.
Prior work in this general field demonstrates that a high level of skill in the art exists for making materials, so that it is not necessary to disclose specific details of the processes of making the materials for use in solar cells. Several representative U.S. patents are exemplary. U.S. Pat. No. 6,281,426 discloses certain structures and compositions without disclosing fabrication techniques and refers to other documents for guidance on growth of materials. U.S. Pat. No. 7,727,795 relates to inverted metamorphic structures for solar cells in which exponential doping is disclosed.
What is needed to continue the progress toward higher efficiency solar cells are designs for multijunction solar cells with four or more subcells that can reach higher efficiencies than can be practically attained with three junction solar cells. It is conventionally assumed that substantially lattice-matched designs are desirable because they have proven reliability and because they use less semiconductor material than metamorphic solar cells, which require relatively thick buffer layers to accommodate differences in the lattice constants of the various materials. It is to be noted that the general understanding of “substantially lattice matched” is that the in-plane lattice constants of the materials in their fully relaxed states differ by less than 0.6% when the materials are present in thicknesses greater than 100 nm. Further, subcells that are substantially lattice matched to each other as used herein means that all materials in the subcells that are present in thicknesses greater than 100 nm have in-plane lattice constants in their fully relaxed states that differ by less than 0.6%.